The present invention relates generally to assembly techniques. The alignment and probing techniques to improve the accuracy of component placement in assembly are described. More particularly, the invention includes methods and structures to detect and improve the component placement accuracy on a target platform by incorporating alignment marks on component and reference marks on target platform with various probing techniques. A set of sensors grouped in an array to form a multiple-sensor probe can detect the deviation of displaced components in assembly. Merely by way of example, the invention has been applied to placing packaged devices onto electronic substrates for the manufacture of electronic systems. But it would be recognized that the invention has a much broader range of applicability.
Electronic devices have proliferated over the years. As the complexity and the operation speed of integrated circuits (IC) increase, it is not unusual to see an increasing number of devices with pin-counts exceeding hundreds or even a thousand. For example, a high-speed design requires more power and ground pins. The differential pairs are replacing the single-end signals at the input and output pins (I/O) of a device to meet the signal integrity requirements. In addition, as system-on-a-chip becomes a reality, more and more pins are added to the device I/O to supports more functions. In generally, many if not all of these tend to increase the number of pins in a packaged device or component.
As the device pin-count increases, the pin pitch of the device tends to decrease to limit the outgrowth of package size. The reduced pin-pitch poses a challenge for the placement equipment to place components accurately on target platform, such as a printed circuit board (PCB), especially if the pin pitch is smaller than 0.5 mm.
The conventional surface-mount equipment uses the Cartesian coordinates at the center of target land pattern as reference point to place a component on PCB. There is no feedback to monitor the accuracy of component placement. Without proper feedback, the accuracy of component placement is uncertain. Actually, the accuracy of component placement is influenced by the imperfectness in package outline, the deviation of component's contact array from ideal grid location, the imperfectness in PCB mounting references, the aging and the intrinsic tolerance of placement equipment, and so on. As the accumulative error is getting closer to the pitch size of contact array, placing a component accurately on PCB is a big challenge.
It is not uncommon to encounter component placement problem in surface mount assembly line, especially for the placement of fine-pitch components. For example, if a BGA component is inaccurately placed on PCB, it could cause the BGA's contact array to deviate from the idea land pattern location, resulting in either inadequate soldering or solder bridging to adjacent pads on PCB. A rework to fix these problems are tedious and expensive. It is even worse for the rework of a pricy, high pin-count component on a high density PCB.
Also, manufacturers frequently use sockets to house high-end, high pin count chips on motherboard. It enables user to choose proper speed grade component or to perform speed upgrade at field. However, there is no handy method for users or manufacturers to monitor if a chip has been properly inserted in the socket or if the chip is in good contact with the receptacle inside the socket.
It is seen that techniques for detecting and improving component placement accuracy and for detecting the contact status are desirable.